The present invention relates to a device transferring method of transferring devices such as semiconductor light emitting devices, and a device arraying method and an image display unit fabricating method for transferring finely formed devices to a wider region by using the device transferring method.
At present, an array of a number of fine devices, electronic parts, electronic devices, or electronic parts formed by burying the above devices or parts in an insulator such as a plastic material are being extensively used for electronic equipment.
The assembly of an image display unit by arraying light emitting devices in a matrix is performed in two manners. For a liquid crystal display (LCD) or a plasma display panel (PDP), the light emitting devices are directly formed on a substrate, and for a light emitting diode display (LED display), single LED packages are arrayed on a substrate. In particular, for an image display unit such as an LCD or PDP, device isolation cannot be performed, so that in general, at the beginning of the fabrication process, devices are formed in such a manner as to be spaced from each other with a pitch equivalent to a pixel pitch of the image display unit.
For an image display unit such as an LCD or PDP, device isolation cannot be performed, so that in general, at the beginning of the fabrication process, devices are formed in such a manner as to be spaced from each other with a pitch equivalent to a pixel pitch of the image display unit.
On the other hand, for an image display unit such as an LED display, LED chips are packaged by taking out LED chips after dicing, and individually connecting the LED chips to external electrodes by wire-bonding or bump-connection using flip-chip. In this case, before or after packaging, the LED chips are arrayed with a pixel pitch of the image display unit; however, such a pixel pitch is independent from an array pitch of the devices at the time of formation of the devices.
Since an LED (Light Emitting. Diode) representative of a light emitting device is expensive, an image display unit using such LEDs can be fabricated at a low cost by producing a large number of LEDs from one wafer. To be more specific, the cost of an image display unit can be lowered by reducing the size of an LED chip from about 300 μm square (ordinary size) to several tens μm square, and producing an image display unit by connecting such small-sized LED chips to each other.
From this viewpoint, there have been known various techniques of transferring devices densely formed on a substrate to a wide region in such a manner that the devices are enlargedly spaced from each other in the wide region, thereby obtaining a relatively large display unit such as an image display unit. For example, U.S. Pat. No. 5,438,241 has disclosed a thin film transfer method, and Japanese Patent Laid-open No. Hei 11-42878 has disclosed a method of forming a transistor array panel for display.
In the transfer method disclosed in U.S. Pat. No. 5,438,241, devices densely formed on a substrate are coarsely re-arrayed by transferring the devices densely formed on the substrate to an extensible substrate provided with an adhesive layer, extending the extensible substrate in the X direction and the Y direction while monitoring a device array pitch and positions of respective devices, and transferring the devices on the extended substrate onto a desired display panel. In the technique disclosed in Japanese Patent Laid-open No. Hei 11-142878, thin film transistors forming a liquid crystal display portion on a first substrate are all transferred onto a second substrate, and the thin film transistors are selectively transferred from the second substrate to a third substrate in such a manner that the transferred transistors are spaced from each other on the third substrate with a pitch corresponding to a pixel pitch.
In the case of producing image display units by the above-described transfer techniques, it is required to selectively, certainly transfer only devices to be transferred, and to efficiently, accurately transfer only devices to be transferred. In general, there has been known a method of using a thermoplastic resin as an adhesive for mounting, microelectronic parts, electronic devices, or electronic parts formed by burying these electronic parts or electronic devices in an insulator such as a plastic material, on a mounting substrate. For example, necessary portions of a mounting substrate are coated with a thermoplastic resin, and electronic parts are placed on the portions of the mounting substrate; and then the entire substrate is heated, to soften the adhesive and cool it, thereby fixing the electronic parts on the substrate. Alternatively, the entire surface of the substrate is coated with a thermoplastic resin, and electronic parts are placed thereon; and then the entire substrate is heated, to soften the adhesive and cool it, thereby fixing the electronic parts on the substrate. In addition, there has been known a method of obtaining the same structure by removing the exposed adhesive by etching or plasma treatment.
In the case of using such a method, however, there arises a problem that since the electronic parts must be placed one by one, the work becomes complicated, and since the entire substrate is heated, there occurs the positional deviation and peeling of other parts. For example, in the case of arranging all of parts on the supply side on a substrate with the array pitch kept as it is, it is possible to use of transferring the parts on the supply side to the substrate. In the case of using a thermoplastic resin for this transfer, the entire substrate is heated by a high frequency heating treatment or exposed to a heating atmosphere, to allow the thermoplastic resin to exhibit an adhesive force stronger than an adhesive force of the parts against the supply side, thereby transferring the parts to the substrate side.
Parts to be transferred and parts not to be transferred can be selectively transferred by using the above transfer method; however, according to the existing technique, it is difficult to heat only the desired parts, and therefore, this method has been not put into practical use. In the case of the overall heating, if an excess portion is coated with a thermoplastic resin, the array positions of parts may be possibly changed by the flow of the thermoplastic resin. Accordingly, in general, it is required to coat portions, at which parts are to be placed, of the substrate with a resin, and therefore, it fails to solve the above-described problem associated with the complicated work. Similarly, there may be considered a method of picking up electronic parts once from a supply source by using an attracting head and placing them on a substrate; however, in this method, if the entire substrate is heated in the case of fixedly transferring the electronic parts from the attracting head to the substrate, there may occur an inconvenience that other parts having been already fixed to the substrate be peeled therefrom.
In the case of performing overall heating by laser irradiation, if either of a thermoplastic resin and each part has a low light absorptivity against laser beams, there occurs a problem that the thermoplastic resin is not heated to a desired temperature. Also, if the parts are taken as heating planes, the parts are required to have a high heat-resistance. Further, in the case of performing overall heating by laser irradiation, it is required to select such a wavelength of laser beams that at least one of a thermoplastic resin, each part, and wiring on the substrate has a high light absorptivity against the laser beams.
For example, there has been known a device transferring method shown in FIGS. 1(a) and 1(b), wherein devices 103 are arrayed on an adhesive layer 102 on a base substrate 101 as shown in FIG. 1(a), and are picked up by using an attracting head 104 as shown in FIG. 1(b), to be transferred to an adhesive layer 106 on another substrate 105.
This method, however, has problems that since a plurality of steps of picking up each device by the attracting head, moving the attracting head, and placing the device to the substrate are required to transfer the devices, the transfer process becomes complicated, and since a plurality of kinds of equipment are required to be provided, the cost is raised, and that since the devices must be picked up one by one for mounting the devices, the mounting work becomes very complicated, and it takes much time to transfer the devices. If it is intended to improve the working efficiency of the mounting machine for shortening the time required to mount the devices, there occurs another problem that the accuracy of arraying of the device at the time of mounting the devices is degraded. Additionally, in the case of using the existing mounting machine, the positioning accuracy at the time of arraying devices has a limitation to about 10 μm, and therefore, it is difficult to further enhance the positioning accuracy by the existing positioning method.